An airborne camera is a mobile camera that is movable in relation to the ground, not only because the aircraft bearing the camera moves in relation to the ground, but also because an operator controls, for example from a station on the ground, the movements of the camera relative to the aircraft, in a manner so as to be able to observe a particular zone being flown over by the latter. The acquired video stream is transmitted, in real-time, to the ground-based station for analysis.
The detection of motion on the ground of vehicles of all types (military vehicle, car, two-wheelers, etc.) is an essential information element to be extracted from the video stream.
The automatic detection of vehicles on the ground and in motion in a video stream that is acquired by a fixed camera, for example mounted rigidly on a mast locally installed in the environment, is already known. The fact that the camera is fixed relative to the ground makes it possible to ignore the backdrop of the scene being observed and to process only the portions of the image that change from one frame of the video stream to the next, and which therefore represent the potential targets.
For a video stream that is acquired by a mobile camera, detection of the portions of the image that change from one frame to the other may be done automatically by effectively implementing a Harris procedure. Such a procedure consists, at the initial stage, of applying an identification algorithm for identifying the noticeable points on an image of the video stream, then, at a subsequent stage, of applying a reconstruction algorithm for associating the noticeable points identified in the image considered, in a manner so as to delimit the portions of the image that correspond to an object being observed. The evolving change in these portions from one image to another makes it possible to determine whether an object is in motion.
However, such an algorithm is not sufficient for distinguishing targets of small sizes, in this case vehicles that are being observed remotely by means of an airborne camera.
But above all, such an algorithm requires a very high amount of computing time, in particular for the association of noticeable points in a manner so as to define the objects. Such a substantial time period for computation is not compatible with carrying out an analysis of the acquired video stream in real-time.
Thus, currently, the video stream that is acquired by an airborne camera is displayed on a screen of the ground-based station and the operator visually analyses the series of successive images in order to try to identify objects in motion. The operator may be forced to remain engaged in the task for the several hours needed to effectively carry out this visual analysis. Given that it would be impossible for the operator to maintain constant and undivided attention, the detection of targets in this way is not always effective.